Field of the Invention
Embodiments described herein relate generally to a multiple charged particle beam apparatus and, for example, relate to a method of correcting spherical aberration when a target object on a stage is irradiated with multiple beams.
Related Art
A lithography technique which leads development of micropatterning of a semiconductor device is a very important process for exclusively generating a pattern in semiconductor manufacturing processes. In recent years, with an increase in integration density of an LSI, a circuit line width required for semiconductor devices is getting smaller year by year. An electron beam pattern writing technique has inherently superior resolution and a pattern is written to wafers and the like using an electron beam.
For example, a pattern writing apparatus using multiple beams is known. Compared with a case in which one electron beam is used, many beams can be emitted at a time by using multiple beams so that throughput can be improved significantly. In such a pattern writing apparatus of multiple beam mode, for example, an electron beam emitted from an electron gun assembly is passed through a mask having a plurality of holes to form multiple beams, each beam is subjected to blanking control, and each beam that is not shielded is shot at a desired position on a target object. In such a pattern writing apparatus of multiple beam mode, electrons scattered when an electron beam is passed through a mask (multiple beam forming aperture plate) having a plurality of holes for forming multiple beams flow into a blanking deflection electrode array that exercises blanking control, posing a problem that an insulator portion of the blanking deflection electrode array is charged. Thus, a method of eliminating scattered electrons entering the blanking deflection electrode array by arranging a doublet lens between the multiple beam forming aperture plate and the blanking deflection electrode array to form a crossover between electromagnetic lenses constituting the doublet lens and arranging an aperture plate (contrast aperture plate) near the crossover is proposed (see Published Unexamined Japanese Patent Application No. 2013-093566 (JP-A-2013-093566), for example).
In multiple beam pattern writing, however, the beam size of whole multiple beams grows and thus, aberration on the optical axis of a crossover image forming system grows. Particularly, if the number of beams is increased, the beam size grows for increased beams and further, aberration grows. Distortion can be decreased by using a doublet lens, but when the beam size of multiple beams is large, a new problem of increased spherical aberration of a crossover image by such a doublet lens arises. Thus, the multiple beam diameter in the crossover position grows. Therefore, it is desirable to suppress distortion of multiple beams and also to decrease spherical aberration itself.